Reference is made to U.K. Patent No. 2,192,460 issued to one of the inventors of the present application relating to a movement sensing detector of the type described herein. Although the sensor as described in the referenced patent works satisfactorily with certain European mattresses, it was found not to be sensitive enough when placed in the catchment area of much larger and thicker sprung mattresses to satisfactorily monitor the well-being of an infant present thereon while the infant is awake or sleeping. There is therefore the need to provide an improved more sensitive sensor capable of operating satisfactorily with larger foam and sprung mattresses.
Several years after the publication of the above referenced U.K. Patent, U.S. Pat. No. 5,271,412 was issued relating to a modified detector particularly useful for apnea (breathing) detection in infants to prevent sudden infant deaths which may occur while the infant is asleep. This improvement was constituted by mounting the electrode side of the piezoelectric transducer on an annular support ring disposed over the bottom plate. However, there are still several disadvantages associated with this modified design. One disadvantage is that the piezoelectric crystal is mounted in an inverted position as compared to the design which was proposed in U.K. Patent 2,192,460, which reduces its sensitivity. Another disadvantage is the assembly of the crystal at a precise location on the annular ring is also time consuming and the detector is more expensive to fabricate. A further disadvantage of this prior art design is that it is already significantly thicker than that proposed in U.K. Patent 2,192,460 and the annular spacer increases this, resulting in sensor pads that are bulky and often not fitting well under the mattress.